The present invention relates to a fixing device used for an electrostatic recording type image forming apparatus such as a copying machine, a facsimile, and a printer. More particularly, it relates to a fixing device of an electromagnetic induction heating system.
Regarding image forming apparatuses such as printers, copying machines, and facsimiles, the demand of the market for energy saving and high speed has increased in recent years. To achieve the demanded performance, it is important that the thermal efficiency of a fixing device used for the image forming apparatus should be improved.
As the fixing device for fixing unfixed toner images, which are formed by a transfer (indirect) system or a direct system using appropriate image forming process means such as electronic photograph recording, electrostatic recording, and magnetic recording, on a recording material such as a recording material sheet, a printing paper, a photosensitive paper, and an electrostatic recording paper, a fixing device of a heat roller system, a film heating system, an electromagnetic induction heating system, or the like has been used widely.
The fixing device of a heat roller system has a heat source such as a tungsten halogen lamp therein, and is basically configured by paired rotating rollers consisting of a fixing roller whose temperature is controlled so as to be a predetermined value and a pressure roller pressed into contact with the fixing roller. A recording material is introduced to the contact portion, what we call a fixing nip portion, of the paired rotating rollers and is conveyed while being held by the fixing nip portion. Unfixed toner images are melted by heat and pressure supplied from the fixing roller and the pressure roller, respectively, to be fixed on the recording material.
Also, the fixing device of a film heating system has been proposed in, for example, JP-A-63-313182 and JP-A-1-263679 specifications or the like.
For this device, a recording material is brought into close contact with a heating element fixedly supported by a support member via a thin fixing film having heat resistance, and the heat of the heating element is supplied to the recording material via the film while the fixing film is slidingly moved with respect to the heating element. In this fixing device, a ceramic heater basically constructed by a ceramic board formed of alumina (Al2O3), aluminum nitride (AlN), or the like having characteristics such as heat resistance, insulating properties, and high thermal conductivity, and a resistance layer, which generates heat by means of carried current, provided on the board can be used as the heating element, and a thin fixing film with low heat capacity can be used. Therefore, for this fixing device, the efficiency of heat transfer is high, the warm-up time can be shortened, and quick start and energy saving can be achieved as compared with the fixing device of a heat roller system.
As the fixing device of an electromagnetic induction heating system, JP-A-11-297462 specification has disclosed a technical idea in which an eddy current is generated in a conductive layer of a fixing roller by an alternating magnetic field to produce Joule""s heat, and the fixing roller is heated by electromagnetic induction by using this Joule""s heat.
The following is a description of the construction of a fixing device of an electromagnetic induction heating system. FIG. 10 is a schematic view of a conventional fixing device of an electromagnetic induction heating system.
The fixing device shown in FIG. 10 includes a fixing roller 21, an exciting coil 22 disposed along the outer peripheral surface of the fixing roller 21, a magnetic element 23 disposed on the outside of the exciting coil 22 so as to cover the exciting coil 22, a pressure roller 24 disposed so as to be pressed into contact with the fixing roller 21, and a temperature sensor 25 for detecting the temperature of the surface of the fixing roller 21.
For the fixing roller 21, a mold release layer formed of, for example, PTFE or PFA with heat resistance, which has a thickness of about 10 to 50 xcexcm, is provided on the surface of a cylinder formed of iron with an outer diameter of 40 mm and a thickness of 0.7 mm.
The pressure roller 24 with an outside diameter of 30 mm is provided with an elastic member such as silicone rubber at the outer periphery of an iron-made core metal, like the fixing roller 21. To enhance the mold release characteristics, a layer formed of, for example, PTFE or PFA with heat resistance, which has a thickness of about 10 to 50 xcexcm, is further provided on the surface of the elastic member.
The fixing roller 21 and the pressure roller 24 are rotatably supported on the housing side of the device, and only the fixing roller 21 is driven. The pressure roller 24 is pressed into contact with the surface of the fixing roller 21, and is rotated in a slave manner by a frictional force at a fixing nip portion N. The pressure roller 24 is pressed in the direction of the axis of rotation of the fixing roller 21 by pressing means (not shown) using a spring or the like.
The exciting coil 22 is disposed along an outer peripheral surface of the fixing roller 21, and is covered with the magnetic element 23. The magnetic element 23 is made of a material with high magnetic permeability and low residual magnetic flux density, such as ferrite and permalloy.
An alternating current of 10 to 100 MHz is applied to the exciting coil 22, and a magnetic field induced by this alternating current causes an eddy current to flow in the conductive layer of the fixing roller 21 to produce Joule""s heat.
The temperature sensor 25 is disposed so as to be in contact with the surface of the fixing roller 21. And, the electric power supplied to the exciting coil 22 is increased or decreased based on the detection signal sent from the temperature sensor 25, by which the temperature of the surface of the fixing roller 21 is automatically controlled so as to be a predetermined fixed temperature.
A recording material 26, which is conveyed while carrying unfixed toner images T, is disposed at a position guided to the nip portion N between the fixing roller 21 and the pressure roller 24 by a conveying guide (not shown).
Thus, the fixing roller 21 is rotationally driven by driving means (not shown), an alternating current is applied to the exciting coil 22 and is introduced to the fixing nip portion N, and the fixing nip portion N is heated to the predetermined temperature. In this state, the recording material 26, which carries unfixed toner images T, is introduced to the fixing nip portion N by being guided by the conveying guide (not shown), and is conveyed along with the rotation of the fixing roller 21, by which the toner images T are melted and fixed on the recording material 26 by the heat of the fixing roller 21 and the nip pressure.
As described above, in the fixing device of an electromagnetic induction heating system, the fixing roller 21 can be heated with high heat transfer by utilizing the eddy current generated by electromagnetic induction. Therefore, this fixing device offers advantages that the warm-up time can be shortened, and quick start and energy saving can be achieved as compared with the fixing device of a film heating system.
Also, JP-A-8-286539 specification has disclosed a configuration in which electromagnetic induction heating means in which an exciting coil is wound along a core material in the direction of the axis of rotation of a rotating heat generating member is provided on the inside of the rotating heat generating member having a conductive layer consisting of a ferromagnetic metallic film etc. formed of nickel, iron, ferromagnetic SUS, nickel-cobalt alloy, or the like.
In the fixing device of an electromagnetic induction heating system disclosed in JP-A-11-297462 specification, although the fixing roller with relatively low heat capacity is used, the heat dissipating area is larger in the end portion than in the central portion in the direction of the axis of rotation of the fixing roller, so that the amount of dissipated heat increases in the end portion of the fixing roller. Therefore, uniform temperature distribution cannot be obtained in the fixing nip portion, and the temperature decreases in the end portion of the fixing roller, so that sufficient thermal energy cannot be supplied to the recording material and the unfixed toner images formed on the recording material in the end portion, which presents a problem that toner is peeled off by the fixing roller, that is, what we call an offset phenomenon takes place.
Also, the fixing device of an electromagnetic induction heating system disclosed in JP-A-8-286539 specification is a system in which a film with very low heat capacity is used as the rotating heat generating member, and the conductive layer of the film is heated by electromagnetic induction heating. Like the above-described fixing device using the fixing roller, the temperature of the end portion decreases as compared with the central portion in the direction of the axis of rotation of the film, so that uniform temperature distribution cannot be obtained in the fixing nip portion, and sufficient thermal energy cannot be supplied in the end portion, which presents a problem that an offset phenomenon takes place.
Further, since this fixing device is configured so that electromagnetic induction heating means such as an exciting coil is provided on the inside of the rotating heat generating member, uniform and efficient heat dissipation of the electromagnetic induction heating means is difficult to do, which presents a problem that the coil itself is heated by self heat generation due to a copper loss of exciting coil.
Accordingly, an object of the present invention is to provide a fixing device of an electromagnetic induction heating system capable of uniforming the temperature distribution in a fixing nip portion in the direction of the axis of rotation of a rotating element and decreasing a rise in temperature of an exciting coil.
To solve the above problems, the present invention provides a fixing device in which a recording material is conveyed by being held by a fixing nip portion, and unfixed toner images on the recording material are melted and fixed, including: a first rotating element of a roller shape; induction heating means, which is provided with an exciting coil wound along the outer peripheral surface or the inner peripheral surface of the first rotating element and having a clearance changing in the direction of the axis of rotation of the first rotating element, for heating the first rotating element by electromagnetic induction; and a pressing member which is pressed into contact with the first rotating element or a second rotating element heated by the first rotating element and is rotated in the forward direction to form the fixing nip portion.
By this configuration, since the clearance of the exciting coil increases from the central portion toward the end portion in the direction of the axis of rotation of the first rotating element, the intensity of magnetic field in the end portion is made higher than in the central portion, so that the heating value increases in the end portion, and thus the temperature distribution in the fixing nip portion can be made uniform.
Also, the present invention provides a fixing device in which a recording material is conveyed by being held by a fixing nip portion, and unfixed toner images on the recording material are melted and fixed, including: a first rotating element of a roller shape; induction heating means, which is provided with an exciting coil wound along the outer peripheral surface or the inner peripheral surface of the first rotating element and having a winding length changing in the direction of the axis of rotation of the first rotating element, for heating the first rotating element by electromagnetic induction; and a pressing member which is pressed into contact with the first rotating element or a second rotating element heated by the first rotating element and is rotated in the forward direction to form the fixing nip portion.
By this configuration, since the winding length of exciting coil increases from the central portion toward the end portion in the direction of the axis of rotation of the first rotating element, the amount of eddy current generated on the surface of the rotating element in the end portion is made larger than the amount of eddy current generated in the central portion, so that the heating value increases in the end portion, and thus the temperature distribution in the fixing nip portion can be made uniform.
Further, the present invention provides a fixing device in which a recording material is conveyed by being held by a fixing nip portion, and unfixed toner images on the recording material are melted and fixed, including: a first rotating element of a roller shape; induction heating means, which is provided with an exciting coil wound along the outer peripheral surface or the inner peripheral surface of the first rotating element and having a cross-sectional area of core material changing in the direction perpendicular to the axis of rotation of the first rotating element, for heating the first rotating element by electromagnetic induction; and a pressing member which is pressed into contact with the first rotating element or a second rotating element heated by the first rotating element and is rotated in the forward direction to form the fixing nip portion.
By this configuration, since the cross-sectional area of core material of the exciting coil increases from the central portion toward the end portion in the direction perpendicular to the axis of rotation of the first rotating element, the absorption efficiency of magnetic field in the end portion is made higher than that in the central portion, so that the heating value increases in the end portion, and thus the temperature distribution in the fixing nip portion can be made uniform.